package org.smile.playground;

import java.util.Scanner;

public class EvenSum {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int i = scanner.nextInt();

        EvenSum evenSum = new EvenSum();
        System.out.println(evenSum.doSum(i));
    }

    public int doSum(int x) {
        if (x < 2) return 0;
        int y = 2;
        int m = 0;
        while (y <= x) {
            m = m + y;
            y = y + 2;
        }
        return m;
    }

}
